This invention relates to liquid metal cooled fast breeder nuclear reactors, and in particular to the structure and construction thereof.
One variation of such nuclear reactors has its core disposed in a pool of liquid metal (generally sodium) which is contained in the inner vessel of a double-walled steel tank supported in a vault and having a roof. Such a nuclear reactor is referred to hereafter as `of the specified type`. The roof provides top shielding and from its outer (upper) side, control, instrumentation and refuelling operations can be effected. The roof also incorporates a rotatable shield to aid refuelling and is necessarily a large and massive structure. In earlier designs of this variation, the roof has been supported from the vault top by a vertical support skirt and the double-walled tank (the inner vessel of which is also known as the primary vessel and the outer vessel being the so-called `guard` vessel) has been suspended from the roof from vertical annular members incorporated therein. It is necessary to construct and position the reactor internals before the roof is fitted, so that joints between the walls of the open-ended primary and guard vessels and the vessel-carrying annular members of the roof are amongst the final operations to be performed, these transition joints being established by welding. It will be appreciated that the welds when established carry the weight of each vessel and, additionally in the case of the primary vessel, its contents of core, coolant and other internals, and are thus in tension. Furthermore the diameter of the roof, which is a large capital item whose size is directly related to its cost, must be large enough to overlap the edges of the vault wall.